From 60e5811b0cb6d9c66997b7d2443731cfac6ec76d Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Thu, 14 May 2026 13:39:37 -0400
Subject: [PATCH 01/17] feat: use batch ingestion for browser analytics
 requests

---
 .changeset/quiet-browsers-batch.md            |  5 ++
 .../browser/functional_tests/mock-server.ts   | 33 ++++++++----
 .../browser/src/__tests__/cookieless.test.ts  |  6 +--
 .../exception-observer.test.ts                |  2 +-
 .../src/__tests__/posthog-core-also.test.ts   | 46 +++++++++++++++-
 .../browser/src/__tests__/request.test.ts     | 20 +++++++
 packages/browser/src/posthog-core.ts          | 29 ++++++++--
 packages/browser/src/request.ts               | 53 ++++++++++++++-----
 packages/browser/src/types.ts                 |  5 ++
 9 files changed, 165 insertions(+), 34 deletions(-)
 create mode 100644 .changeset/quiet-browsers-batch.md

diff --git a/.changeset/quiet-browsers-batch.md b/.changeset/quiet-browsers-batch.md
new file mode 100644
index 0000000000..56f16999f3
--- /dev/null
+++ b/.changeset/quiet-browsers-batch.md
@@ -0,0 +1,5 @@
+---
+'posthog-js': minor
+---
+
+Send regular browser analytics requests through batch ingestion with HTTP gzip encoding while preserving legacy sendBeacon delivery.
diff --git a/packages/browser/functional_tests/mock-server.ts b/packages/browser/functional_tests/mock-server.ts
index 15d74d3411..2317dadc59 100644
--- a/packages/browser/functional_tests/mock-server.ts
+++ b/packages/browser/functional_tests/mock-server.ts
@@ -5,6 +5,7 @@ import { setupServer } from 'msw/node'
 import { RestContext } from 'msw'
 import { RestRequest } from 'msw'
 import { decompressSync, strFromU8 } from 'fflate'
+import { isArray } from '@posthog/core'
 
 // the request bodies in a store that we can inspect within tests.
 const capturedRequests: { '/e/': any[]; '/engage/': any[]; '/flags/': any[] } = {
@@ -16,25 +17,32 @@ const capturedRequests: { '/e/': any[]; '/engage/': any[]; '/flags/': any[] } =
 const handleRequest = (group: string) => (req: RestRequest, res: ResponseComposition, ctx: RestContext) => {
     let body = req.body
 
-    if (typeof body === 'string') {
-        try {
-            const b64Encoded = req.url.href.includes('compression=base64')
-            const gzipCompressed = req.url.href.includes('compression=gzip-js')
+    try {
+        const b64Encoded = req.url.href.includes('compression=base64')
+        const gzipCompressed =
+            req.url.href.includes('compression=gzip-js') || req.headers.get('content-encoding') === 'gzip'
+        if (gzipCompressed) {
+            const data = new Uint8Array(req._body)
+            const decoded = strFromU8(decompressSync(data))
+            body = JSON.parse(decoded)
+        } else if (typeof body === 'string') {
             if (b64Encoded) {
                 body = JSON.parse(Buffer.from(decodeURIComponent(body.split('=')[1]), 'base64').toString())
-            } else if (gzipCompressed) {
-                const data = new Uint8Array(req._body)
-                const decoded = strFromU8(decompressSync(data))
-                body = JSON.parse(decoded)
+            } else if (body[0] === '{' || body[0] === '[') {
+                body = JSON.parse(body)
             } else {
                 body = JSON.parse(decodeURIComponent(body.split('=')[1]))
             }
-        } catch {
-            return res(ctx.status(500))
         }
+    } catch {
+        return res(ctx.status(500))
     }
 
-    capturedRequests[group] = [...(capturedRequests[group] || []), body]
+    if (group === '/batch/' && body && typeof body === 'object' && isArray(body.batch)) {
+        capturedRequests['/e/'] = [...capturedRequests['/e/'], ...body.batch]
+    } else {
+        capturedRequests[group] = [...(capturedRequests[group] || []), body]
+    }
 
     return res(ctx.json({}))
 }
@@ -43,6 +51,9 @@ const server = setupServer(
     rest.post('http://localhost/e/', (req, res, ctx) => {
         return handleRequest('/e/')(req, res, ctx)
     }),
+    rest.post('http://localhost/batch/', (req, res, ctx) => {
+        return handleRequest('/batch/')(req, res, ctx)
+    }),
     rest.post('http://localhost/engage/', (req, res, ctx) => {
         return handleRequest('/engage/')(req, res, ctx)
     }),
diff --git a/packages/browser/src/__tests__/cookieless.test.ts b/packages/browser/src/__tests__/cookieless.test.ts
index b2c9e529c7..66dc57db8e 100644
--- a/packages/browser/src/__tests__/cookieless.test.ts
+++ b/packages/browser/src/__tests__/cookieless.test.ts
@@ -374,13 +374,13 @@ describe('cookieless', () => {
 
             posthog.opt_in_capturing()
             expect(mockedFetch).toBeCalledTimes(3) // flags + opt in + pageview
-            expect(JSON.parse(mockedFetch.mock.calls[1][1].body).event).toEqual('$opt_in')
-            expect(JSON.parse(mockedFetch.mock.calls[2][1].body).event).toEqual('$pageview')
+            expect(JSON.parse(mockedFetch.mock.calls[1][1].body).batch[0].event).toEqual('$opt_in')
+            expect(JSON.parse(mockedFetch.mock.calls[2][1].body).batch[0].event).toEqual('$pageview')
 
             posthog.capture('custom event')
             jest.advanceTimersByTime(5000) // flush the batch queue (3s interval) without triggering 5-min remote config refresh
             expect(mockedFetch).toBeCalledTimes(4) // flags + opt in + pageview + custom event
-            expect(JSON.parse(mockedFetch.mock.calls[3][1].body)[0].event).toEqual('custom event')
+            expect(JSON.parse(mockedFetch.mock.calls[3][1].body).batch[0].event).toEqual('custom event')
         })
     })
 })
diff --git a/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts b/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
index cffc38da15..7757e2f310 100644
--- a/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
+++ b/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
@@ -146,7 +146,7 @@ describe('Exception Observer', () => {
 
             expect(sendRequestSpy).toHaveBeenCalled()
             const request = sendRequestSpy.mock.calls[0][0]
-            expect(request.url).toBe('http://localhost/e/?ip=0')
+            expect(request.url).toBe('http://localhost/e/')
             expect(request.data).toMatchObject({
                 event: '$exception',
                 properties: {
diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index bc19952194..1cb1b91f07 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -6,8 +6,9 @@ import { uuidv7 } from '../uuidv7'
 import { isUndefined } from '@posthog/core'
 import { ENABLE_PERSON_PROCESSING, USER_STATE } from '../constants'
 import { createPosthogInstance, defaultPostHog } from './helpers/posthog-instance'
-import { PostHogConfig, RemoteConfig } from '../types'
+import { Compression, PostHogConfig, RemoteConfig } from '../types'
 import { PostHog } from '../posthog-core'
+import * as requestModule from '../request'
 import { PostHogPersistence } from '../posthog-persistence'
 import { SessionIdManager } from '../sessionid'
 import { RequestQueue } from '../request-queue'
@@ -351,6 +352,49 @@ describe('posthog core', () => {
             )
         })
 
+        it('sends non-beacon analytics requests to /batch/ with HTTP gzip semantics', () => {
+            const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
+            const posthog = posthogWith({ ...defaultConfig, request_batching: false })
+            posthog._onRemoteConfig({ supportedCompression: [Compression.GZipJS] } as RemoteConfig)
+            requestSpy.mockClear()
+
+            posthog.capture('event-name', { foo: 'bar', length: 0 })
+
+            expect(requestSpy).toHaveBeenCalledWith(
+                expect.objectContaining({
+                    url: 'https://us.i.posthog.com/batch/?ip=0',
+                    compression: Compression.GZipJS,
+                    _compressionEncoding: 'content-encoding',
+                    data: expect.objectContaining({
+                        api_key: 'testtoken',
+                        sent_at: '2020-01-01T00:00:00.000Z',
+                        batch: [expect.objectContaining({ event: 'event-name' })],
+                    }),
+                })
+            )
+            requestSpy.mockRestore()
+        })
+
+        it('keeps sendBeacon analytics requests on the legacy /e/ protocol', () => {
+            const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
+            const posthog = posthogWith({ ...defaultConfig, request_batching: false })
+            posthog._onRemoteConfig({ supportedCompression: [Compression.GZipJS] } as RemoteConfig)
+            requestSpy.mockClear()
+
+            posthog.capture('event-name', { foo: 'bar', length: 0 }, { transport: 'sendBeacon' } as any)
+
+            expect(requestSpy).toHaveBeenCalledWith(
+                expect.objectContaining({
+                    url: 'https://us.i.posthog.com/e/?ip=0',
+                    compression: Compression.GZipJS,
+                    data: expect.objectContaining({ event: 'event-name' }),
+                    transport: 'sendBeacon',
+                })
+            )
+            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_compressionEncoding')
+            requestSpy.mockRestore()
+        })
+
         it('does not allow you to set complex current url', () => {
             const posthog = posthogWith(defaultConfig, defaultOverrides)
             const captureResult = posthog.capture('event-name', { $current_url: new URL('https://app.posthog.com/s/') })
diff --git a/packages/browser/src/__tests__/request.test.ts b/packages/browser/src/__tests__/request.test.ts
index 2ce7f76823..ea636c4f64 100644
--- a/packages/browser/src/__tests__/request.test.ts
+++ b/packages/browser/src/__tests__/request.test.ts
@@ -469,12 +469,32 @@ describe('request', () => {
                 "
             `)
 
+                expect(mockedXHR.setRequestHeader).toHaveBeenCalledWith('Content-Type', 'text/plain')
+                expect(mockedXHR.setRequestHeader).not.toHaveBeenCalledWith('Content-Encoding', 'gzip')
                 expect(mockedXHR.setRequestHeader).not.toHaveBeenCalledWith(
                     'Content-Type',
                     'application/x-www-form-urlencoded'
                 )
             })
 
+            it('should use HTTP content encoding for gzip when requested', async () => {
+                request(
+                    createRequest({
+                        url: 'https://any.posthog-instance.com/',
+                        method: 'POST',
+                        compression: Compression.GZipJS,
+                        _compressionEncoding: 'content-encoding',
+                        data: { foo: 'bar' },
+                    } as any)
+                )
+
+                expect(mockedXHR.open.mock.calls[0][1]).not.toContain('compression=gzip-js')
+                expect(mockedXHR.send).toHaveBeenCalledTimes(1)
+                expect(mockedXHR.send.mock.calls[0][0]).toBeInstanceOf(ArrayBuffer)
+                expect(mockedXHR.setRequestHeader).toHaveBeenCalledWith('Content-Type', 'application/json')
+                expect(mockedXHR.setRequestHeader).toHaveBeenCalledWith('Content-Encoding', 'gzip')
+            })
+
             it('converts bigint properties to string without throwing', () => {
                 request(
                     createRequest({
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index cd908dbdf3..0200dffb45 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -38,7 +38,7 @@ import {
 import { ProductTourEventName, ProductTourEventProperties } from './posthog-product-tours-types'
 import { RateLimiter } from './rate-limiter'
 import { RemoteConfigLoader } from './remote-config'
-import { extendURLParams, request, SUPPORTS_REQUEST } from './request'
+import { COMPRESSION_ENCODING_CONTENT_ENCODING, extendURLParams, request, SUPPORTS_REQUEST } from './request'
 import { DEFAULT_FLUSH_INTERVAL_MS, RequestQueue } from './request-queue'
 import { RetryQueue } from './retry-queue'
 import { ScrollManager } from './scroll-manager'
@@ -108,6 +108,7 @@ import {
     isEmptyObject,
     isObject,
     isBoolean,
+    currentISOTime,
 } from '@posthog/core'
 import { uuidv7 } from './uuidv7'
 import { ExternalIntegrations } from './extensions/external-integration'
@@ -158,6 +159,8 @@ const instances: Record<string, PostHog> = {}
 // calls into push() calls, which would otherwise cause infinite recursion.
 let _executeArrayDepth = 0
 
+const COMPRESSION_BEST_AVAILABLE = 'best-available'
+
 const __NOOP = () => {}
 const CONSENT_COOKIELESS_WARN = 'Consent opt in/out is not valid with cookieless_mode="always" and will be ignored'
 const SURVEYS_NOT_AVAILABLE = 'Surveys module not available'
@@ -1010,7 +1013,9 @@ export class PostHog implements PostHogInterface {
         this._retryQueue?.unload()
     }
 
-    _send_request(options: QueuedRequestWithOptions): void {
+    _send_request(_options: QueuedRequestWithOptions): void {
+        const options = { ..._options }
+
         if (!this.__loaded) {
             return
         }
@@ -1025,6 +1030,22 @@ export class PostHog implements PostHogInterface {
         }
 
         options.transport = options.transport || this.config.api_transport
+        options.compression =
+            options.compression === COMPRESSION_BEST_AVAILABLE ? this.compression : options.compression
+
+        const isAnalyticsRequest = options.url === this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
+        if (isAnalyticsRequest && options.transport !== 'sendBeacon' && options.data) {
+            options.url = this.requestRouter.endpointFor('api', '/batch/')
+            options.data = {
+                api_key: this.config.token,
+                batch: isArray(options.data) ? options.data : [options.data],
+                sent_at: currentISOTime(),
+            }
+            options._compressionEncoding =
+                options.compression === Compression.GZipJS ? COMPRESSION_ENCODING_CONTENT_ENCODING : undefined
+            options.compression = options.compression === Compression.GZipJS ? Compression.GZipJS : undefined
+        }
+
         options.url = extendURLParams(options.url, {
             // Whether to detect ip info or not
             ip: this.config.ip ? 1 : 0,
@@ -1033,7 +1054,6 @@ export class PostHog implements PostHogInterface {
             ...this.config.request_headers,
             ...options.headers,
         }
-        options.compression = options.compression === 'best-available' ? this.compression : options.compression
         options.disableXHRCredentials = this.config.__preview_disable_xhr_credentials
         if (this.config.__preview_disable_beacon) {
             options.disableTransport = ['sendBeacon']
@@ -1362,8 +1382,9 @@ export class PostHog implements PostHogInterface {
             method: 'POST',
             url: options?._url ?? this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint),
             data,
-            compression: 'best-available',
+            compression: COMPRESSION_BEST_AVAILABLE,
             batchKey: options?._batchKey,
+            transport: options?.transport,
         }
 
         if (this.config.request_batching && (!options || options?._batchKey) && !options?.send_instantly) {
diff --git a/packages/browser/src/request.ts b/packages/browser/src/request.ts
index b5ccb178ab..f166faed8a 100644
--- a/packages/browser/src/request.ts
+++ b/packages/browser/src/request.ts
@@ -26,6 +26,8 @@ export const SUPPORTS_REQUEST = !!XMLHttpRequest || !!fetch
 const CONTENT_TYPE_PLAIN = 'text/plain'
 const CONTENT_TYPE_JSON = 'application/json'
 const CONTENT_TYPE_FORM = 'application/x-www-form-urlencoded'
+const CONTENT_ENCODING_GZIP = 'gzip'
+export const COMPRESSION_ENCODING_CONTENT_ENCODING = 'content-encoding'
 const SIXTY_FOUR_KILOBYTES = 64 * 1024
 /*
  fetch will fail if we request keepalive with a body greater than 64kb
@@ -53,6 +55,7 @@ const removeURLParam = (url: string, param: string): string => {
 
 type EncodedBody = {
     contentType: string
+    contentEncoding?: string
     body: string | BlobPart | ArrayBuffer
     estimatedSize: number
 }
@@ -62,6 +65,10 @@ type EncodedRequest = {
     encodedBody?: EncodedBody
 }
 
+const useHTTPContentEncoding = (options: RequestWithEncodedBody): boolean => {
+    return options._compressionEncoding === COMPRESSION_ENCODING_CONTENT_ENCODING
+}
+
 /**
  * Extends a URL with additional query parameters
  * @param url - The URL to extend
@@ -115,8 +122,10 @@ const encodePostData = (options: RequestWithEncodedBody): EncodedBody | undefine
 
     if (compression === Compression.GZipJS) {
         const gzipData = gzipSync(strToU8(jsonStringify(data)), { mtime: 0 })
+        const useContentEncoding = useHTTPContentEncoding(options)
         return {
-            contentType: CONTENT_TYPE_PLAIN,
+            contentType: useContentEncoding ? CONTENT_TYPE_JSON : CONTENT_TYPE_PLAIN,
+            contentEncoding: useContentEncoding ? CONTENT_ENCODING_GZIP : undefined,
             body: gzipData.buffer.slice(gzipData.byteOffset, gzipData.byteOffset + gzipData.byteLength) as ArrayBuffer,
             estimatedSize: gzipData.byteLength,
         }
@@ -183,7 +192,8 @@ const preEncodeAsync = async (options: RequestWithEncodedBody): Promise<RequestW
     return {
         ...options,
         _encodedBody: {
-            contentType: CONTENT_TYPE_PLAIN,
+            contentType: useHTTPContentEncoding(options) ? CONTENT_TYPE_JSON : CONTENT_TYPE_PLAIN,
+            contentEncoding: useHTTPContentEncoding(options) ? CONTENT_ENCODING_GZIP : undefined,
             body,
             estimatedSize: body.byteLength,
         },
@@ -194,7 +204,7 @@ const xhr = (options: RequestWithOptions) => {
     const req = new XMLHttpRequest!()
     const { url, encodedBody } = encodePostDataSafely(options)
     req.open(options.method || 'GET', url, true)
-    const { contentType, body } = encodedBody ?? {}
+    const { contentType, contentEncoding, body } = encodedBody ?? {}
 
     each(options.headers, function (headerValue, headerName) {
         req.setRequestHeader(headerName, headerValue)
@@ -204,6 +214,10 @@ const xhr = (options: RequestWithOptions) => {
         req.setRequestHeader('Content-Type', contentType)
     }
 
+    if (contentEncoding) {
+        req.setRequestHeader('Content-Encoding', contentEncoding)
+    }
+
     if (options.timeout) {
         req.timeout = options.timeout
     }
@@ -235,7 +249,7 @@ const xhr = (options: RequestWithOptions) => {
 
 const _fetch = (options: RequestWithOptions) => {
     const { url, encodedBody } = encodePostDataSafely(options)
-    const { contentType, body, estimatedSize } = encodedBody ?? {}
+    const { contentType, contentEncoding, body, estimatedSize } = encodedBody ?? {}
 
     // eslint-disable-next-line compat/compat
     const headers = new Headers()
@@ -247,6 +261,10 @@ const _fetch = (options: RequestWithOptions) => {
         headers.append('Content-Type', contentType)
     }
 
+    if (contentEncoding) {
+        headers.append('Content-Encoding', contentEncoding)
+    }
+
     let aborter: { signal: any; timeout: ReturnType<typeof setTimeout> } | null = null
 
     if (AbortController) {
@@ -323,11 +341,11 @@ const _sendBeacon = (options: RequestWithOptions) => {
     }
 }
 
-const buildRequestURL = (url: string, compression?: RequestWithOptions['compression']): string => {
+const buildRequestURL = (url: string, options: RequestWithEncodedBody): string => {
     return extendURLParams(url, {
         _: new Date().getTime().toString(),
         ver: Config.JS_SDK_VERSION,
-        compression,
+        compression: useHTTPContentEncoding(options) ? undefined : options.compression,
     })
 }
 
@@ -364,26 +382,29 @@ export const request = (_options: RequestWithOptions) => {
     const options: RequestWithEncodedBody = { ..._options }
     options.timeout = options.timeout || 60000
 
-    options.url = buildRequestURL(options.url, options.compression)
-
-    const transport = options.transport ?? 'fetch'
+    const requestedTransport = options.transport ?? 'fetch'
 
     const availableTransports = AVAILABLE_TRANSPORTS.filter(
         (t) => !options.disableTransport || !t.transport || !options.disableTransport.includes(t.transport)
     )
 
-    const transportMethod =
-        find(availableTransports, (t) => t.transport === transport)?.method ?? availableTransports[0].method
+    const transport = find(availableTransports, (t) => t.transport === requestedTransport) ?? availableTransports[0]
+    const transportMethod = transport?.method
 
     if (!transportMethod) {
         throw new Error('No available transport method')
     }
 
+    if (transport.transport === 'sendBeacon') {
+        options._compressionEncoding = undefined
+    }
+    options.url = buildRequestURL(options.url, options)
+
     // For non-sendBeacon transports, use async native CompressionStream when available
     // to avoid blocking the main thread with fflate's synchronous gzip (which can take 300ms+).
     // sendBeacon must remain synchronous as it's used during page unload.
     if (
-        transport !== 'sendBeacon' &&
+        transport.transport !== 'sendBeacon' &&
         options.data &&
         options.compression === Compression.GZipJS &&
         !!CompressionStream &&
@@ -396,10 +417,14 @@ export const request = (_options: RequestWithOptions) => {
             .catch((error) => {
                 if (isNativeAsyncGzipReadError(error)) {
                     nativeAsyncGzipDisabled = true
-                    transportMethod({
+                    const uncompressedOptions = {
                         ...options,
                         compression: undefined,
-                        url: buildRequestURL(_options.url, undefined),
+                        _compressionEncoding: undefined,
+                    }
+                    transportMethod({
+                        ...uncompressedOptions,
+                        url: buildRequestURL(_options.url, uncompressedOptions),
                     })
                     return
                 }
diff --git a/packages/browser/src/types.ts b/packages/browser/src/types.ts
index 620e532b21..798b658355 100644
--- a/packages/browser/src/types.ts
+++ b/packages/browser/src/types.ts
@@ -192,6 +192,11 @@ export interface RequestWithOptions {
     disableTransport?: ('XHR' | 'fetch' | 'sendBeacon')[]
     disableXHRCredentials?: boolean
     compression?: Compression | 'best-available'
+    /**
+     * Internal: when set, gzip is signaled using the standard HTTP
+     * Content-Encoding header instead of PostHog's legacy query-param protocol.
+     */
+    _compressionEncoding?: 'content-encoding'
     fetchOptions?: {
         cache?: RequestInit['cache']
         next?: NextOptions

From c844223a54a83da63cdb4ddf9bd296ab6a2608c5 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Mon, 18 May 2026 18:16:52 +0200
Subject: [PATCH 02/17] fix browser batch retry review feedback

---
 .../src/__tests__/posthog-core-also.test.ts   | 30 +++++++++++++++++--
 .../browser/src/__tests__/request.test.ts     |  2 +-
 packages/browser/src/posthog-core.ts          | 12 ++++----
 packages/browser/src/request.ts               |  7 ++---
 packages/browser/src/types.ts                 |  4 +--
 5 files changed, 40 insertions(+), 15 deletions(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index 1cb1b91f07..a34e643337 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -364,7 +364,7 @@ describe('posthog core', () => {
                 expect.objectContaining({
                     url: 'https://us.i.posthog.com/batch/?ip=0',
                     compression: Compression.GZipJS,
-                    _compressionEncoding: 'content-encoding',
+                    _useContentEncoding: true,
                     data: expect.objectContaining({
                         api_key: 'testtoken',
                         sent_at: '2020-01-01T00:00:00.000Z',
@@ -375,6 +375,32 @@ describe('posthog core', () => {
             requestSpy.mockRestore()
         })
 
+        it('sends retried analytics requests with query params to /batch/', () => {
+            const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
+            const posthog = posthogWith({ ...defaultConfig, request_batching: false })
+            posthog._onRemoteConfig({ supportedCompression: [Compression.GZipJS] } as RemoteConfig)
+            requestSpy.mockClear()
+
+            posthog._send_request({
+                url: 'https://us.i.posthog.com/e/?retry_count=1',
+                data: { event: 'event-name' },
+                compression: Compression.GZipJS,
+            })
+
+            expect(requestSpy).toHaveBeenCalledWith(
+                expect.objectContaining({
+                    url: 'https://us.i.posthog.com/batch/?retry_count=1&ip=0',
+                    compression: Compression.GZipJS,
+                    _useContentEncoding: true,
+                    data: expect.objectContaining({
+                        api_key: 'testtoken',
+                        batch: [expect.objectContaining({ event: 'event-name' })],
+                    }),
+                })
+            )
+            requestSpy.mockRestore()
+        })
+
         it('keeps sendBeacon analytics requests on the legacy /e/ protocol', () => {
             const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
             const posthog = posthogWith({ ...defaultConfig, request_batching: false })
@@ -391,7 +417,7 @@ describe('posthog core', () => {
                     transport: 'sendBeacon',
                 })
             )
-            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_compressionEncoding')
+            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_useContentEncoding')
             requestSpy.mockRestore()
         })
 
diff --git a/packages/browser/src/__tests__/request.test.ts b/packages/browser/src/__tests__/request.test.ts
index ea636c4f64..3be2b8d938 100644
--- a/packages/browser/src/__tests__/request.test.ts
+++ b/packages/browser/src/__tests__/request.test.ts
@@ -483,7 +483,7 @@ describe('request', () => {
                         url: 'https://any.posthog-instance.com/',
                         method: 'POST',
                         compression: Compression.GZipJS,
-                        _compressionEncoding: 'content-encoding',
+                        _useContentEncoding: true,
                         data: { foo: 'bar' },
                     } as any)
                 )
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index 0200dffb45..174f0580ad 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -38,7 +38,7 @@ import {
 import { ProductTourEventName, ProductTourEventProperties } from './posthog-product-tours-types'
 import { RateLimiter } from './rate-limiter'
 import { RemoteConfigLoader } from './remote-config'
-import { COMPRESSION_ENCODING_CONTENT_ENCODING, extendURLParams, request, SUPPORTS_REQUEST } from './request'
+import { extendURLParams, request, SUPPORTS_REQUEST } from './request'
 import { DEFAULT_FLUSH_INTERVAL_MS, RequestQueue } from './request-queue'
 import { RetryQueue } from './retry-queue'
 import { ScrollManager } from './scroll-manager'
@@ -1033,17 +1033,17 @@ export class PostHog implements PostHogInterface {
         options.compression =
             options.compression === COMPRESSION_BEST_AVAILABLE ? this.compression : options.compression
 
-        const isAnalyticsRequest = options.url === this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
+        const analyticsEndpoint = this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
+        const isAnalyticsRequest = options.url === analyticsEndpoint || options.url.startsWith(`${analyticsEndpoint}?`)
         if (isAnalyticsRequest && options.transport !== 'sendBeacon' && options.data) {
-            options.url = this.requestRouter.endpointFor('api', '/batch/')
+            options.url = options.url.replace(analyticsEndpoint, this.requestRouter.endpointFor('api', '/batch/'))
             options.data = {
                 api_key: this.config.token,
                 batch: isArray(options.data) ? options.data : [options.data],
                 sent_at: currentISOTime(),
             }
-            options._compressionEncoding =
-                options.compression === Compression.GZipJS ? COMPRESSION_ENCODING_CONTENT_ENCODING : undefined
-            options.compression = options.compression === Compression.GZipJS ? Compression.GZipJS : undefined
+            options._useContentEncoding = options.compression === Compression.GZipJS
+            options.compression = options.compression === Compression.GZipJS ? options.compression : undefined
         }
 
         options.url = extendURLParams(options.url, {
diff --git a/packages/browser/src/request.ts b/packages/browser/src/request.ts
index f166faed8a..e173d97a49 100644
--- a/packages/browser/src/request.ts
+++ b/packages/browser/src/request.ts
@@ -27,7 +27,6 @@ const CONTENT_TYPE_PLAIN = 'text/plain'
 const CONTENT_TYPE_JSON = 'application/json'
 const CONTENT_TYPE_FORM = 'application/x-www-form-urlencoded'
 const CONTENT_ENCODING_GZIP = 'gzip'
-export const COMPRESSION_ENCODING_CONTENT_ENCODING = 'content-encoding'
 const SIXTY_FOUR_KILOBYTES = 64 * 1024
 /*
  fetch will fail if we request keepalive with a body greater than 64kb
@@ -66,7 +65,7 @@ type EncodedRequest = {
 }
 
 const useHTTPContentEncoding = (options: RequestWithEncodedBody): boolean => {
-    return options._compressionEncoding === COMPRESSION_ENCODING_CONTENT_ENCODING
+    return !!options._useContentEncoding
 }
 
 /**
@@ -396,7 +395,7 @@ export const request = (_options: RequestWithOptions) => {
     }
 
     if (transport.transport === 'sendBeacon') {
-        options._compressionEncoding = undefined
+        options._useContentEncoding = undefined
     }
     options.url = buildRequestURL(options.url, options)
 
@@ -420,7 +419,7 @@ export const request = (_options: RequestWithOptions) => {
                     const uncompressedOptions = {
                         ...options,
                         compression: undefined,
-                        _compressionEncoding: undefined,
+                        _useContentEncoding: undefined,
                     }
                     transportMethod({
                         ...uncompressedOptions,
diff --git a/packages/browser/src/types.ts b/packages/browser/src/types.ts
index 798b658355..fb43b1a6df 100644
--- a/packages/browser/src/types.ts
+++ b/packages/browser/src/types.ts
@@ -193,10 +193,10 @@ export interface RequestWithOptions {
     disableXHRCredentials?: boolean
     compression?: Compression | 'best-available'
     /**
-     * Internal: when set, gzip is signaled using the standard HTTP
+     * Internal: when true, gzip is signaled using the standard HTTP
      * Content-Encoding header instead of PostHog's legacy query-param protocol.
      */
-    _compressionEncoding?: 'content-encoding'
+    _useContentEncoding?: boolean
     fetchOptions?: {
         cache?: RequestInit['cache']
         next?: NextOptions

From 878638793c304af26e1c9e7ddad9ced6c588ceca Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Mon, 18 May 2026 18:27:56 +0200
Subject: [PATCH 03/17] omit unused query params for browser batch requests

---
 .../src/__tests__/posthog-core-also.test.ts        |  4 ++--
 packages/browser/src/posthog-core.ts               | 14 +++++++++-----
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index a34e643337..47043b535d 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -362,7 +362,7 @@ describe('posthog core', () => {
 
             expect(requestSpy).toHaveBeenCalledWith(
                 expect.objectContaining({
-                    url: 'https://us.i.posthog.com/batch/?ip=0',
+                    url: 'https://us.i.posthog.com/batch/',
                     compression: Compression.GZipJS,
                     _useContentEncoding: true,
                     data: expect.objectContaining({
@@ -389,7 +389,7 @@ describe('posthog core', () => {
 
             expect(requestSpy).toHaveBeenCalledWith(
                 expect.objectContaining({
-                    url: 'https://us.i.posthog.com/batch/?retry_count=1&ip=0',
+                    url: 'https://us.i.posthog.com/batch/',
                     compression: Compression.GZipJS,
                     _useContentEncoding: true,
                     data: expect.objectContaining({
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index 174f0580ad..2525cd9c37 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -1035,8 +1035,10 @@ export class PostHog implements PostHogInterface {
 
         const analyticsEndpoint = this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
         const isAnalyticsRequest = options.url === analyticsEndpoint || options.url.startsWith(`${analyticsEndpoint}?`)
+        let isBatchAnalyticsRequest = false
         if (isAnalyticsRequest && options.transport !== 'sendBeacon' && options.data) {
-            options.url = options.url.replace(analyticsEndpoint, this.requestRouter.endpointFor('api', '/batch/'))
+            options.url = this.requestRouter.endpointFor('api', '/batch/')
+            isBatchAnalyticsRequest = true
             options.data = {
                 api_key: this.config.token,
                 batch: isArray(options.data) ? options.data : [options.data],
@@ -1046,10 +1048,12 @@ export class PostHog implements PostHogInterface {
             options.compression = options.compression === Compression.GZipJS ? options.compression : undefined
         }
 
-        options.url = extendURLParams(options.url, {
-            // Whether to detect ip info or not
-            ip: this.config.ip ? 1 : 0,
-        })
+        if (!isBatchAnalyticsRequest) {
+            options.url = extendURLParams(options.url, {
+                // Whether to detect ip info or not
+                ip: this.config.ip ? 1 : 0,
+            })
+        }
         options.headers = {
             ...this.config.request_headers,
             ...options.headers,

From fca8c78742d6cbc54be0022c3b9f9df483e999f1 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 10:38:33 +0200
Subject: [PATCH 04/17] route beacon analytics requests through batch

---
 .../browser/src/__tests__/posthog-core-also.test.ts   | 11 +++++++----
 packages/browser/src/posthog-core.ts                  |  2 +-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index 47043b535d..62c3dd21f3 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -401,7 +401,7 @@ describe('posthog core', () => {
             requestSpy.mockRestore()
         })
 
-        it('keeps sendBeacon analytics requests on the legacy /e/ protocol', () => {
+        it('sends sendBeacon analytics requests to /batch/', () => {
             const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
             const posthog = posthogWith({ ...defaultConfig, request_batching: false })
             posthog._onRemoteConfig({ supportedCompression: [Compression.GZipJS] } as RemoteConfig)
@@ -411,13 +411,16 @@ describe('posthog core', () => {
 
             expect(requestSpy).toHaveBeenCalledWith(
                 expect.objectContaining({
-                    url: 'https://us.i.posthog.com/e/?ip=0',
+                    url: 'https://us.i.posthog.com/batch/',
                     compression: Compression.GZipJS,
-                    data: expect.objectContaining({ event: 'event-name' }),
+                    _useContentEncoding: true,
+                    data: expect.objectContaining({
+                        api_key: 'testtoken',
+                        batch: [expect.objectContaining({ event: 'event-name' })],
+                    }),
                     transport: 'sendBeacon',
                 })
             )
-            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_useContentEncoding')
             requestSpy.mockRestore()
         })
 
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index 2525cd9c37..4237bb13f7 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -1036,7 +1036,7 @@ export class PostHog implements PostHogInterface {
         const analyticsEndpoint = this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
         const isAnalyticsRequest = options.url === analyticsEndpoint || options.url.startsWith(`${analyticsEndpoint}?`)
         let isBatchAnalyticsRequest = false
-        if (isAnalyticsRequest && options.transport !== 'sendBeacon' && options.data) {
+        if (isAnalyticsRequest && options.data) {
             options.url = this.requestRouter.endpointFor('api', '/batch/')
             isBatchAnalyticsRequest = true
             options.data = {

From 50258eb574b3736a4a3744fb27ccb7c6b6d489bf Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 10:44:30 +0200
Subject: [PATCH 05/17] default browser analytics endpoint to batch

---
 .../browser/src/__tests__/posthog-core-also.test.ts    | 10 +++++-----
 packages/browser/src/posthog-core.ts                   |  2 +-
 packages/browser/src/types.ts                          |  2 +-
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index 62c3dd21f3..2aa6247d2e 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -302,14 +302,14 @@ describe('posthog core', () => {
             expect(captureResult.properties).toEqual(expect.objectContaining({ foo: 'bar', length: 0 }))
         })
 
-        it('sends payloads to /e/ by default', () => {
+        it('sends payloads to /batch/ by default', () => {
             const posthog = posthogWith({ ...defaultConfig, request_batching: false }, defaultOverrides)
 
             posthog.capture('event-name', { foo: 'bar', length: 0 })
 
             expect(posthog._send_request).toHaveBeenCalledWith(
                 expect.objectContaining({
-                    url: 'https://us.i.posthog.com/e/',
+                    url: 'https://us.i.posthog.com/batch/',
                 })
             )
         })
@@ -382,7 +382,7 @@ describe('posthog core', () => {
             requestSpy.mockClear()
 
             posthog._send_request({
-                url: 'https://us.i.posthog.com/e/?retry_count=1',
+                url: 'https://us.i.posthog.com/batch/?retry_count=1',
                 data: { event: 'event-name' },
                 compression: Compression.GZipJS,
             })
@@ -474,12 +474,12 @@ describe('posthog core', () => {
             expect(posthog.compression).toEqual(undefined)
         })
 
-        it('defaults to /e if no endpoint is given', () => {
+        it('defaults to /batch if no endpoint is given', () => {
             const posthog = posthogWith({})
 
             posthog._onRemoteConfig({} as RemoteConfig)
 
-            expect(posthog.analyticsDefaultEndpoint).toEqual('/e/')
+            expect(posthog.analyticsDefaultEndpoint).toEqual('/batch/')
         })
 
         it('uses the specified analytics endpoint if given', () => {
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index 4237bb13f7..d1ff789131 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -450,7 +450,7 @@ export class PostHog implements PostHogInterface {
         this.sentryIntegration = (options?: SentryIntegrationOptions) => sentryIntegration(this, options)
         this.__request_queue = []
         this.__loaded = false
-        this.analyticsDefaultEndpoint = '/e/'
+        this.analyticsDefaultEndpoint = '/batch/'
         this._initialPageviewCaptured = false
         this._visibilityStateListener = null
         this._initialPersonProfilesConfig = null
diff --git a/packages/browser/src/types.ts b/packages/browser/src/types.ts
index fb43b1a6df..0e9020731d 100644
--- a/packages/browser/src/types.ts
+++ b/packages/browser/src/types.ts
@@ -311,7 +311,7 @@ export interface RemoteConfig {
     /**
      * Whether we should use a custom endpoint for analytics
      *
-     * @default { endpoint: "/e" }
+     * @default { endpoint: "/batch" }
      */
     analytics?: {
         endpoint?: string

From 1df1f02bde6a2b9693a5efb2dbce2f21e7bdd93b Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 10:53:11 +0200
Subject: [PATCH 06/17] remove deprecated ip query param

---
 .../src/__tests__/extensions/replay/config.test.ts   | 12 ++++++------
 packages/browser/src/posthog-core.ts                 | 10 +---------
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/packages/browser/src/__tests__/extensions/replay/config.test.ts b/packages/browser/src/__tests__/extensions/replay/config.test.ts
index 100dec5520..2977389ce5 100644
--- a/packages/browser/src/__tests__/extensions/replay/config.test.ts
+++ b/packages/browser/src/__tests__/extensions/replay/config.test.ts
@@ -79,21 +79,21 @@ describe('config', () => {
                 ],
                 [
                     {
-                        name: 'https://app.posthog.com/s/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/s/?ver=123',
                     },
                     undefined,
                     undefined,
                 ],
                 [
                     {
-                        name: 'https://app.posthog.com/e/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/e/?ver=123',
                     },
                     undefined,
                     undefined,
                 ],
                 [
                     {
-                        name: 'https://app.posthog.com/i/v0/e/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/i/v0/e/?ver=123',
                     },
                     undefined,
                     undefined,
@@ -101,7 +101,7 @@ describe('config', () => {
                 [
                     {
                         // even an imaginary future world of rust session replay capture
-                        name: 'https://app.posthog.com/i/v0/s/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/i/v0/s/?ver=123',
                     },
                     undefined,
                     undefined,
@@ -109,7 +109,7 @@ describe('config', () => {
                 [
                     {
                         // using a relative path as a reverse proxy api host
-                        name: 'https://app.posthog.com/ingest/s/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/ingest/s/?ver=123',
                     },
                     undefined,
                     '/ingest',
@@ -117,7 +117,7 @@ describe('config', () => {
                 [
                     {
                         // using a reverse proxy with a path
-                        name: 'https://app.posthog.com/ingest/s/?ip=0&ver=123',
+                        name: 'https://app.posthog.com/ingest/s/?ver=123',
                     },
                     undefined,
                     'https://app.posthog.com/ingest',
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index d1ff789131..c143b24169 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -38,7 +38,7 @@ import {
 import { ProductTourEventName, ProductTourEventProperties } from './posthog-product-tours-types'
 import { RateLimiter } from './rate-limiter'
 import { RemoteConfigLoader } from './remote-config'
-import { extendURLParams, request, SUPPORTS_REQUEST } from './request'
+import { request, SUPPORTS_REQUEST } from './request'
 import { DEFAULT_FLUSH_INTERVAL_MS, RequestQueue } from './request-queue'
 import { RetryQueue } from './retry-queue'
 import { ScrollManager } from './scroll-manager'
@@ -1035,10 +1035,8 @@ export class PostHog implements PostHogInterface {
 
         const analyticsEndpoint = this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
         const isAnalyticsRequest = options.url === analyticsEndpoint || options.url.startsWith(`${analyticsEndpoint}?`)
-        let isBatchAnalyticsRequest = false
         if (isAnalyticsRequest && options.data) {
             options.url = this.requestRouter.endpointFor('api', '/batch/')
-            isBatchAnalyticsRequest = true
             options.data = {
                 api_key: this.config.token,
                 batch: isArray(options.data) ? options.data : [options.data],
@@ -1048,12 +1046,6 @@ export class PostHog implements PostHogInterface {
             options.compression = options.compression === Compression.GZipJS ? options.compression : undefined
         }
 
-        if (!isBatchAnalyticsRequest) {
-            options.url = extendURLParams(options.url, {
-                // Whether to detect ip info or not
-                ip: this.config.ip ? 1 : 0,
-            })
-        }
         options.headers = {
             ...this.config.request_headers,
             ...options.headers,

From 7daa329ff40629e319668bd1b20cdb1d7b1bbb47 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 10:59:05 +0200
Subject: [PATCH 07/17] omit timestamp query param for analytics batches

---
 packages/browser/src/__tests__/request.test.ts | 16 ++++++++++++++++
 packages/browser/src/posthog-core.ts           |  3 +++
 packages/browser/src/request.ts                |  2 +-
 packages/browser/src/types.ts                  |  5 +++++
 4 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/packages/browser/src/__tests__/request.test.ts b/packages/browser/src/__tests__/request.test.ts
index 3be2b8d938..091f39cecb 100644
--- a/packages/browser/src/__tests__/request.test.ts
+++ b/packages/browser/src/__tests__/request.test.ts
@@ -598,6 +598,22 @@ describe('request', () => {
             `)
             })
 
+            it('should omit timestamp query param when requested', () => {
+                request(
+                    createRequest({
+                        url: 'https://any.posthog-instance.com/',
+                        method: 'POST',
+                        data: { foo: 'bar' },
+                        _skipTimestampQueryParam: true,
+                    } as any)
+                )
+
+                expect(mockedNavigator?.sendBeacon).toHaveBeenCalledWith(
+                    'https://any.posthog-instance.com/?ver=1.23.45&beacon=1',
+                    expect.any(Blob)
+                )
+            })
+
             it('should not call sendBeacon when body is undefined', () => {
                 request(
                     createRequest({
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index c143b24169..fa206d3a63 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -1043,6 +1043,9 @@ export class PostHog implements PostHogInterface {
                 sent_at: currentISOTime(),
             }
             options._useContentEncoding = options.compression === Compression.GZipJS
+            // /batch/ carries sent_at in the JSON body. The backend prefers that value over the legacy `_`
+            // query param, so don't send redundant cache-buster timestamp metadata for analytics batches.
+            options._skipTimestampQueryParam = true
             options.compression = options.compression === Compression.GZipJS ? options.compression : undefined
         }
 
diff --git a/packages/browser/src/request.ts b/packages/browser/src/request.ts
index e173d97a49..d0549b7c70 100644
--- a/packages/browser/src/request.ts
+++ b/packages/browser/src/request.ts
@@ -342,7 +342,7 @@ const _sendBeacon = (options: RequestWithOptions) => {
 
 const buildRequestURL = (url: string, options: RequestWithEncodedBody): string => {
     return extendURLParams(url, {
-        _: new Date().getTime().toString(),
+        _: options._skipTimestampQueryParam ? undefined : new Date().getTime().toString(),
         ver: Config.JS_SDK_VERSION,
         compression: useHTTPContentEncoding(options) ? undefined : options.compression,
     })
diff --git a/packages/browser/src/types.ts b/packages/browser/src/types.ts
index 0e9020731d..9b399c78b5 100644
--- a/packages/browser/src/types.ts
+++ b/packages/browser/src/types.ts
@@ -197,6 +197,11 @@ export interface RequestWithOptions {
      * Content-Encoding header instead of PostHog's legacy query-param protocol.
      */
     _useContentEncoding?: boolean
+    /**
+     * Internal: when true, skips the `_` query parameter because the request body
+     * already carries equivalent timestamp metadata.
+     */
+    _skipTimestampQueryParam?: boolean
     fetchOptions?: {
         cache?: RequestInit['cache']
         next?: NextOptions

From 441f598bf3cebea6d4c8dde8156169004b126866 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 11:11:12 +0200
Subject: [PATCH 08/17] honor configured analytics endpoint payload format

---
 .../src/__tests__/posthog-core-also.test.ts   | 23 +++++++++++++++++++
 packages/browser/src/posthog-core.ts          |  8 +++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index 2aa6247d2e..c2eef51112 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -327,6 +327,29 @@ describe('posthog core', () => {
             )
         })
 
+        it('sends legacy analytics payload shape when remote config selects a legacy endpoint', () => {
+            const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
+            const posthog = posthogWith({ ...defaultConfig, request_batching: false })
+            posthog._onRemoteConfig({
+                analytics: { endpoint: '/i/v0/e/' },
+                supportedCompression: [Compression.GZipJS],
+            } as RemoteConfig)
+            requestSpy.mockClear()
+
+            posthog.capture('event-name', { foo: 'bar', length: 0 })
+
+            expect(requestSpy).toHaveBeenCalledWith(
+                expect.objectContaining({
+                    url: 'https://us.i.posthog.com/i/v0/e/',
+                    compression: Compression.GZipJS,
+                    data: expect.objectContaining({ event: 'event-name' }),
+                })
+            )
+            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_useContentEncoding')
+            expect(requestSpy.mock.calls[0][0]).not.toHaveProperty('_skipTimestampQueryParam')
+            requestSpy.mockRestore()
+        })
+
         it('sends payloads to overriden endpoint if given', () => {
             const posthog = posthogWith({ ...defaultConfig, request_batching: false }, defaultOverrides)
 
diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index fa206d3a63..e595f60345 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -1033,10 +1033,10 @@ export class PostHog implements PostHogInterface {
         options.compression =
             options.compression === COMPRESSION_BEST_AVAILABLE ? this.compression : options.compression
 
-        const analyticsEndpoint = this.requestRouter.endpointFor('api', this.analyticsDefaultEndpoint)
-        const isAnalyticsRequest = options.url === analyticsEndpoint || options.url.startsWith(`${analyticsEndpoint}?`)
-        if (isAnalyticsRequest && options.data) {
-            options.url = this.requestRouter.endpointFor('api', '/batch/')
+        const batchEndpoint = this.requestRouter.endpointFor('api', '/batch/')
+        const isBatchAnalyticsRequest = options.url === batchEndpoint || options.url.startsWith(`${batchEndpoint}?`)
+        if (isBatchAnalyticsRequest && options.data) {
+            options.url = batchEndpoint
             options.data = {
                 api_key: this.config.token,
                 batch: isArray(options.data) ? options.data : [options.data],

From 61d361123d999556f3635b82ad980f8a438a9207 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 11:11:45 +0200
Subject: [PATCH 09/17] document remote analytics endpoint default

---
 packages/browser/src/types.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/browser/src/types.ts b/packages/browser/src/types.ts
index 9b399c78b5..d46a67677b 100644
--- a/packages/browser/src/types.ts
+++ b/packages/browser/src/types.ts
@@ -314,7 +314,10 @@ export interface RemoteConfig {
     capturePerformance?: boolean | PerformanceCaptureConfig
 
     /**
-     * Whether we should use a custom endpoint for analytics
+     * Whether we should use a custom endpoint for analytics.
+     *
+     * The SDK defaults to `/batch/`, but remote config may currently return
+     * `/i/v0/e/` for legacy analytics ingestion.
      *
      * @default { endpoint: "/batch" }
      */

From 4b502aa3f30a4c5086471445fa55cece47d5c65b Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 11:27:06 +0200
Subject: [PATCH 10/17] update browser terser mangled names

---
 packages/browser/terser-mangled-names.json | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packages/browser/terser-mangled-names.json b/packages/browser/terser-mangled-names.json
index 3c49c246da..12a6c5a176 100644
--- a/packages/browser/terser-mangled-names.json
+++ b/packages/browser/terser-mangled-names.json
@@ -535,6 +535,7 @@
         "_showTicketList",
         "_shutdown",
         "_shutdownOnce",
+        "_skipTimestampQueryParam",
         "_sortSurveysByAppearanceDelay",
         "_spanContext",
         "_staleCacheRefreshTriggered",
@@ -599,6 +600,7 @@
         "_urlTriggerMatching",
         "_urlTriggerStatus",
         "_urlTriggers",
+        "_useContentEncoding",
         "_validateEmail",
         "_validateIdentifyId",
         "_validateSampleRate",

From 77c40cd360385515ead62d4bd62b63385a5c5bef Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 11:28:11 +0200
Subject: [PATCH 11/17] update browser analytics endpoint test expectations

---
 .../exception-autocapture/exception-observer.test.ts |  2 +-
 .../src/__tests__/posthog-core.beforeSend.test.ts    | 12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts b/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
index 7757e2f310..b745c784a9 100644
--- a/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
+++ b/packages/browser/src/__tests__/extensions/exception-autocapture/exception-observer.test.ts
@@ -146,7 +146,7 @@ describe('Exception Observer', () => {
 
             expect(sendRequestSpy).toHaveBeenCalled()
             const request = sendRequestSpy.mock.calls[0][0]
-            expect(request.url).toBe('http://localhost/e/')
+            expect(request.url).toBe('http://localhost/batch/')
             expect(request.data).toMatchObject({
                 event: '$exception',
                 properties: {
diff --git a/packages/browser/src/__tests__/posthog-core.beforeSend.test.ts b/packages/browser/src/__tests__/posthog-core.beforeSend.test.ts
index 4d9641eebd..87a796e49d 100644
--- a/packages/browser/src/__tests__/posthog-core.beforeSend.test.ts
+++ b/packages/browser/src/__tests__/posthog-core.beforeSend.test.ts
@@ -72,7 +72,8 @@ describe('posthog core - before send', () => {
             compression: 'best-available',
             data: capturedData,
             method: 'POST',
-            url: 'https://us.i.posthog.com/e/',
+            transport: undefined,
+            url: 'https://us.i.posthog.com/batch/',
         })
     })
 
@@ -108,7 +109,8 @@ describe('posthog core - before send', () => {
             compression: 'best-available',
             data: capturedData[0],
             method: 'POST',
-            url: 'https://us.i.posthog.com/e/',
+            transport: undefined,
+            url: 'https://us.i.posthog.com/batch/',
         })
     })
 
@@ -130,7 +132,8 @@ describe('posthog core - before send', () => {
             compression: 'best-available',
             data: capturedData,
             method: 'POST',
-            url: 'https://us.i.posthog.com/e/',
+            transport: undefined,
+            url: 'https://us.i.posthog.com/batch/',
         })
     })
 
@@ -152,7 +155,8 @@ describe('posthog core - before send', () => {
             compression: 'best-available',
             data: capturedData,
             method: 'POST',
-            url: 'https://us.i.posthog.com/e/',
+            transport: undefined,
+            url: 'https://us.i.posthog.com/batch/',
         })
         expect(mockLogger.warn).toHaveBeenCalledWith(
             `Event '${eventName}' has no properties after beforeSend function, this is likely an error.`

From 2f8153684ee7993758829f54eaf6a9b0cdbd9e95 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 11:46:59 +0200
Subject: [PATCH 12/17] fix sendBeacon content encoding test expectation

---
 .../src/__tests__/posthog-core-also.test.ts    |  1 -
 packages/browser/src/__tests__/request.test.ts | 18 ++++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index 3667a60476..fd2535397e 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -436,7 +436,6 @@ describe('posthog core', () => {
                 expect.objectContaining({
                     url: 'https://us.i.posthog.com/batch/',
                     compression: Compression.GZipJS,
-                    _useContentEncoding: true,
                     data: expect.objectContaining({
                         api_key: 'testtoken',
                         batch: [expect.objectContaining({ event: 'event-name' })],
diff --git a/packages/browser/src/__tests__/request.test.ts b/packages/browser/src/__tests__/request.test.ts
index 091f39cecb..4787e1f9d6 100644
--- a/packages/browser/src/__tests__/request.test.ts
+++ b/packages/browser/src/__tests__/request.test.ts
@@ -614,6 +614,24 @@ describe('request', () => {
                 )
             })
 
+            it('uses query-param compression for sendBeacon when content encoding is requested', () => {
+                request(
+                    createRequest({
+                        url: 'https://any.posthog-instance.com/batch/',
+                        method: 'POST',
+                        compression: Compression.GZipJS,
+                        data: { foo: 'bar' },
+                        _useContentEncoding: true,
+                        _skipTimestampQueryParam: true,
+                    } as any)
+                )
+
+                expect(mockedNavigator?.sendBeacon).toHaveBeenCalledWith(
+                    'https://any.posthog-instance.com/batch/?ver=1.23.45&compression=gzip-js&beacon=1',
+                    expect.any(Blob)
+                )
+            })
+
             it('should not call sendBeacon when body is undefined', () => {
                 request(
                     createRequest({

From 596de304bb9e14c8e6a965be44cd1963466fb3a8 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 12:38:34 +0200
Subject: [PATCH 13/17] avoid startsWith for IE compatibility

---
 packages/browser/src/posthog-core.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/browser/src/posthog-core.ts b/packages/browser/src/posthog-core.ts
index c32561c3ea..66a92e1005 100644
--- a/packages/browser/src/posthog-core.ts
+++ b/packages/browser/src/posthog-core.ts
@@ -1035,7 +1035,7 @@ export class PostHog implements PostHogInterface {
             options.compression === COMPRESSION_BEST_AVAILABLE ? this.compression : options.compression
 
         const batchEndpoint = this.requestRouter.endpointFor('api', '/batch/')
-        const isBatchAnalyticsRequest = options.url === batchEndpoint || options.url.startsWith(`${batchEndpoint}?`)
+        const isBatchAnalyticsRequest = options.url === batchEndpoint || options.url.indexOf(`${batchEndpoint}?`) === 0
         if (isBatchAnalyticsRequest && options.data) {
             options.url = batchEndpoint
             options.data = {

From e3cc0bd8553517fe4fada900d180462b01d8619b Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 12:59:41 +0200
Subject: [PATCH 14/17] chore: disallow startsWith in browser sdk

---
 .../external/components/RichContent.tsx       | 26 +++++-----
 .../conversations/external/url-utils.ts       |  2 +-
 .../product-tours/product-tours.tsx           | 11 +++--
 .../replay/external/network-plugin.ts         |  6 ++-
 packages/browser/src/extensions/surveys.tsx   |  2 +-
 .../surveys/surveys-extension-utils.tsx       |  4 +-
 packages/browser/src/posthog-exceptions.ts    |  2 +-
 packages/browser/src/posthog-surveys.ts       |  2 +-
 .../no-browser-startswith.js                  | 43 +++++++++++++++++
 .../no-browser-startswith.test.js             | 48 +++++++++++++++++++
 10 files changed, 120 insertions(+), 26 deletions(-)
 create mode 100644 tooling/eslint-plugin-posthog-js/no-browser-startswith.js
 create mode 100644 tooling/eslint-plugin-posthog-js/no-browser-startswith.test.js

diff --git a/packages/browser/src/extensions/conversations/external/components/RichContent.tsx b/packages/browser/src/extensions/conversations/external/components/RichContent.tsx
index e0e5bf82a9..d2196e1345 100644
--- a/packages/browser/src/extensions/conversations/external/components/RichContent.tsx
+++ b/packages/browser/src/extensions/conversations/external/components/RichContent.tsx
@@ -46,10 +46,10 @@ function sanitizeUrl(url: string): string | undefined {
 
     // Block dangerous protocols
     if (
-        normalizedForCheck.startsWith('javascript:') ||
-        normalizedForCheck.startsWith('vbscript:') ||
-        normalizedForCheck.startsWith('data:') ||
-        normalizedForCheck.startsWith('file:')
+        normalizedForCheck.indexOf('javascript:') === 0 ||
+        normalizedForCheck.indexOf('vbscript:') === 0 ||
+        normalizedForCheck.indexOf('data:') === 0 ||
+        normalizedForCheck.indexOf('file:') === 0
     ) {
         return undefined
     }
@@ -57,14 +57,14 @@ function sanitizeUrl(url: string): string | undefined {
     // Allow relative URLs (check against trimmed URL, not normalized)
     // Note: We explicitly check for '//' first to block protocol-relative URLs (e.g., //evil.com)
     // which could be used to load content from attacker-controlled domains
-    if (trimmedUrl.startsWith('//')) {
+    if (trimmedUrl.indexOf('//') === 0) {
         return undefined
     }
     if (
-        trimmedUrl.startsWith('/') ||
-        trimmedUrl.startsWith('./') ||
-        trimmedUrl.startsWith('../') ||
-        trimmedUrl.startsWith('#')
+        trimmedUrl.indexOf('/') === 0 ||
+        trimmedUrl.indexOf('./') === 0 ||
+        trimmedUrl.indexOf('../') === 0 ||
+        trimmedUrl.indexOf('#') === 0
     ) {
         return trimmedUrl
     }
@@ -72,10 +72,10 @@ function sanitizeUrl(url: string): string | undefined {
     // Allow safe absolute URLs
     const lowerUrl = trimmedUrl.toLowerCase()
     if (
-        lowerUrl.startsWith('http://') ||
-        lowerUrl.startsWith('https://') ||
-        lowerUrl.startsWith('mailto:') ||
-        lowerUrl.startsWith('tel:')
+        lowerUrl.indexOf('http://') === 0 ||
+        lowerUrl.indexOf('https://') === 0 ||
+        lowerUrl.indexOf('mailto:') === 0 ||
+        lowerUrl.indexOf('tel:') === 0
     ) {
         return trimmedUrl
     }
diff --git a/packages/browser/src/extensions/conversations/external/url-utils.ts b/packages/browser/src/extensions/conversations/external/url-utils.ts
index 622eb64c23..95419e5b12 100644
--- a/packages/browser/src/extensions/conversations/external/url-utils.ts
+++ b/packages/browser/src/extensions/conversations/external/url-utils.ts
@@ -36,7 +36,7 @@ export function isCurrentDomainAllowed(domains: string[] | undefined): boolean {
             return false
         }
 
-        if (allowedHostname.startsWith('*.')) {
+        if (allowedHostname.indexOf('*.') === 0) {
             const pattern = allowedHostname.slice(2)
             return currentHostname.endsWith(`.${pattern}`) || currentHostname === pattern
         }
diff --git a/packages/browser/src/extensions/product-tours/product-tours.tsx b/packages/browser/src/extensions/product-tours/product-tours.tsx
index ebc3889dfb..de610a7c08 100644
--- a/packages/browser/src/extensions/product-tours/product-tours.tsx
+++ b/packages/browser/src/extensions/product-tours/product-tours.tsx
@@ -969,7 +969,7 @@ export class ProductTourManager {
             return true
         }
         const isFeatureEnabled = !!this._instance.featureFlags?.isFeatureEnabled(flagKey, {
-            send_event: !flagKey.startsWith(PRODUCT_TOUR_TARGETING_FLAG_PREFIX),
+            send_event: flagKey.indexOf(PRODUCT_TOUR_TARGETING_FLAG_PREFIX) !== 0,
         })
         let flagVariantCheck = true
         if (flagVariant) {
@@ -1132,10 +1132,11 @@ export class ProductTourManager {
         for (let i = 0; i < storage.length; i++) {
             const key = storage.key(i)
             if (
-                key?.startsWith(TOUR_SHOWN_KEY_PREFIX) ||
-                key?.startsWith(TOUR_COMPLETED_KEY_PREFIX) ||
-                key?.startsWith(TOUR_DISMISSED_KEY_PREFIX) ||
-                key?.startsWith(LAST_SEEN_TOUR_DATE_KEY_PREFIX)
+                key &&
+                (key.indexOf(TOUR_SHOWN_KEY_PREFIX) === 0 ||
+                    key.indexOf(TOUR_COMPLETED_KEY_PREFIX) === 0 ||
+                    key.indexOf(TOUR_DISMISSED_KEY_PREFIX) === 0 ||
+                    key.indexOf(LAST_SEEN_TOUR_DATE_KEY_PREFIX) === 0)
             ) {
                 keysToRemove.push(key)
             }
diff --git a/packages/browser/src/extensions/replay/external/network-plugin.ts b/packages/browser/src/extensions/replay/external/network-plugin.ts
index e4b9dcfdbb..3458647561 100644
--- a/packages/browser/src/extensions/replay/external/network-plugin.ts
+++ b/packages/browser/src/extensions/replay/external/network-plugin.ts
@@ -134,7 +134,7 @@ export function shouldRecordBody({
     function isBlobURL(url: string | URL | RequestInfo) {
         try {
             if (typeof url === 'string') {
-                return url.startsWith('blob:')
+                return url.indexOf('blob:') === 0
             }
             if (url instanceof URL) {
                 return url.protocol === 'blob:'
@@ -476,7 +476,9 @@ function _checkForCannotReadResponseBody({
     // `get` and `has` are case-insensitive
     // but return the header value with the casing that was supplied
     const contentType = r.headers.get('Content-Type')?.toLowerCase()
-    const contentTypeIsDenied = contentTypePrefixDenyList.some((prefix) => contentType?.startsWith(prefix))
+    const contentTypeIsDenied = contentTypePrefixDenyList.some(
+        (prefix) => !!contentType && contentType.indexOf(prefix) === 0
+    )
     if (contentType && contentTypeIsDenied) {
         return `Content-Type ${contentType} is not supported`
     }
diff --git a/packages/browser/src/extensions/surveys.tsx b/packages/browser/src/extensions/surveys.tsx
index e02f25a6c8..88888fc298 100644
--- a/packages/browser/src/extensions/surveys.tsx
+++ b/packages/browser/src/extensions/surveys.tsx
@@ -556,7 +556,7 @@ export class SurveyManager {
             return true
         }
         const isFeatureEnabled = !!this._posthog.featureFlags?.isFeatureEnabled(flagKey, {
-            send_event: !flagKey.startsWith(SURVEY_TARGETING_FLAG_PREFIX),
+            send_event: flagKey.indexOf(SURVEY_TARGETING_FLAG_PREFIX) !== 0,
         })
         let flagVariantCheck = true
         if (flagVariant) {
diff --git a/packages/browser/src/extensions/surveys/surveys-extension-utils.tsx b/packages/browser/src/extensions/surveys/surveys-extension-utils.tsx
index 6c37fd0d5a..6540aa3aa2 100644
--- a/packages/browser/src/extensions/surveys/surveys-extension-utils.tsx
+++ b/packages/browser/src/extensions/surveys/surveys-extension-utils.tsx
@@ -306,7 +306,7 @@ function nameToHex(name: string) {
 }
 
 export function hex2rgb(c: string): string {
-    if (c.startsWith('#')) {
+    if (c.indexOf('#') === 0) {
         let hexColor = c.replace(/^#/, '')
         // Handle 3-character shorthand (e.g., #111 -> #111111, #abc -> #aabbcc)
         if (/^[0-9A-Fa-f]{3}$/.test(hexColor)) {
@@ -337,7 +337,7 @@ export function getContrastingTextColor(color: string = defaultSurveyAppearance.
     if (color[0] === '#') {
         rgb = hex2rgb(color)
     }
-    if (color.startsWith('rgb')) {
+    if (color.indexOf('rgb') === 0) {
         rgb = color
     }
     // otherwise it's a color name
diff --git a/packages/browser/src/posthog-exceptions.ts b/packages/browser/src/posthog-exceptions.ts
index 1ebea24be6..1eff33f7a8 100644
--- a/packages/browser/src/posthog-exceptions.ts
+++ b/packages/browser/src/posthog-exceptions.ts
@@ -236,7 +236,7 @@ export class PostHogExceptions implements Extension {
 
     private _isExtensionException(exceptionList: ErrorTracking.ExceptionList): boolean {
         const frames = exceptionList.flatMap((e) => e.stacktrace?.frames ?? [])
-        return frames.some((f) => f.filename && f.filename.startsWith('chrome-extension://'))
+        return frames.some((f) => f.filename && f.filename.indexOf('chrome-extension://') === 0)
     }
 
     private _isPostHogException(exceptionList: ErrorTracking.ExceptionList): boolean {
diff --git a/packages/browser/src/posthog-surveys.ts b/packages/browser/src/posthog-surveys.ts
index a4bed6d51e..a6e59dd39b 100644
--- a/packages/browser/src/posthog-surveys.ts
+++ b/packages/browser/src/posthog-surveys.ts
@@ -77,7 +77,7 @@ export class PostHogSurveys implements Extension {
         const surveyKeys = []
         for (let i = 0; i < localStorage.length; i++) {
             const key = localStorage.key(i)
-            if (key?.startsWith(SURVEY_SEEN_PREFIX) || key?.startsWith(SURVEY_IN_PROGRESS_PREFIX)) {
+            if (key && (key.indexOf(SURVEY_SEEN_PREFIX) === 0 || key.indexOf(SURVEY_IN_PROGRESS_PREFIX) === 0)) {
                 surveyKeys.push(key)
             }
         }
diff --git a/tooling/eslint-plugin-posthog-js/no-browser-startswith.js b/tooling/eslint-plugin-posthog-js/no-browser-startswith.js
new file mode 100644
index 0000000000..5c5aea0f50
--- /dev/null
+++ b/tooling/eslint-plugin-posthog-js/no-browser-startswith.js
@@ -0,0 +1,43 @@
+function isBrowserSdkSourceFile(filename) {
+    const normalizedFilename = filename.replace(/\\/g, '/')
+    const isBrowserSrc =
+        normalizedFilename.includes('/packages/browser/src/') ||
+        normalizedFilename.indexOf('packages/browser/src/') === 0
+    return isBrowserSrc && !normalizedFilename.includes('/__tests__/')
+}
+
+function isStartsWithCall(callee) {
+    if (!callee) {
+        return false
+    }
+
+    if (callee.type === 'ChainExpression') {
+        return isStartsWithCall(callee.expression)
+    }
+
+    if (callee.type !== 'MemberExpression') {
+        return false
+    }
+
+    return callee.property?.type === 'Identifier' && callee.property.name === 'startsWith'
+}
+
+module.exports = {
+    create(context) {
+        if (!isBrowserSdkSourceFile(context.getFilename())) {
+            return {}
+        }
+
+        return {
+            CallExpression(node) {
+                if (isStartsWithCall(node.callee)) {
+                    context.report({
+                        node,
+                        message:
+                            'Do not use String.prototype.startsWith in the browser SDK — IE11 does not support it. Use indexOf(...) === 0 instead.',
+                    })
+                }
+            },
+        }
+    },
+}
diff --git a/tooling/eslint-plugin-posthog-js/no-browser-startswith.test.js b/tooling/eslint-plugin-posthog-js/no-browser-startswith.test.js
new file mode 100644
index 0000000000..16b1df2c88
--- /dev/null
+++ b/tooling/eslint-plugin-posthog-js/no-browser-startswith.test.js
@@ -0,0 +1,48 @@
+const noBrowserStartsWith = require('./no-browser-startswith')
+const { RuleTester } = require('eslint')
+
+const ruleTester = new RuleTester({
+    parserOptions: {
+        ecmaVersion: 2020,
+        sourceType: 'module',
+    },
+    env: {
+        browser: true,
+    },
+})
+
+const errorMessage =
+    'Do not use String.prototype.startsWith in the browser SDK — IE11 does not support it. Use indexOf(...) === 0 instead.'
+
+ruleTester.run('no-browser-startswith', noBrowserStartsWith, {
+    valid: [
+        {
+            code: "value.indexOf('prefix') === 0",
+            filename: '/project/packages/browser/src/something.ts',
+        },
+        {
+            code: "value.indexOf('prefix') === 0",
+            filename: 'packages/browser/src/something.ts',
+        },
+        {
+            code: "value.startsWith('prefix')",
+            filename: '/project/packages/browser/src/__tests__/something.test.ts',
+        },
+        {
+            code: "value.startsWith('prefix')",
+            filename: '/project/packages/node/src/something.ts',
+        },
+    ],
+    invalid: [
+        {
+            code: "value.startsWith('prefix')",
+            filename: '/project/packages/browser/src/something.ts',
+            errors: [{ message: errorMessage }],
+        },
+        {
+            code: "value?.startsWith('prefix')",
+            filename: '/project/packages/browser/src/something.ts',
+            errors: [{ message: errorMessage }],
+        },
+    ],
+})

From 505f21f6dec0b428a7b3373dee5f62eb17dd7d2e Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 17:03:48 +0200
Subject: [PATCH 15/17] test: update playwright mocks for batch ingestion

---
 packages/browser/playwright/fixtures/network.ts      |  2 +-
 packages/browser/playwright/mocked/capture.spec.ts   | 12 +++++++-----
 .../browser/playwright/mocked/retry-queue.spec.ts    |  6 +++---
 .../session-recording-network-recorder.spec.ts       |  5 +----
 .../browser/playwright/mocked/slim-bundle.spec.ts    |  2 +-
 .../mocked/utils/posthog-playwright-test-base.ts     |  2 +-
 6 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/packages/browser/playwright/fixtures/network.ts b/packages/browser/playwright/fixtures/network.ts
index b6fb2d762f..223792e432 100644
--- a/packages/browser/playwright/fixtures/network.ts
+++ b/packages/browser/playwright/fixtures/network.ts
@@ -113,7 +113,7 @@ export class NetworkPage {
     }
 
     async mockIngestion() {
-        await this.page.route('**/e/**', async (route) => {
+        await this.page.route(/\/(?:e|batch)\//, async (route) => {
             await route.fulfill({
                 headers: { loaded: 'mock captured' },
                 json: {},
diff --git a/packages/browser/playwright/mocked/capture.spec.ts b/packages/browser/playwright/mocked/capture.spec.ts
index b7f718b1ce..98098efd6a 100644
--- a/packages/browser/playwright/mocked/capture.spec.ts
+++ b/packages/browser/playwright/mocked/capture.spec.ts
@@ -43,7 +43,7 @@ test.describe('event capture', () => {
         const captureRequests: Request[] = []
 
         page.on('request', (request) => {
-            if (request.url().includes('/e/') && request.method() === 'POST') {
+            if (request.url().includes('/batch/') && request.method() === 'POST') {
                 captureRequests.push(request)
             }
         })
@@ -55,14 +55,16 @@ test.describe('event capture', () => {
         await pollUntilCondition(page, () => captureRequests.length > 0)
         expect(captureRequests.length).toEqual(1)
         const captureRequest = captureRequests[0]
-        expect(captureRequest.headers()['content-type']).toEqual('text/plain')
-        expect(captureRequest.url()).toMatch(/gzip/)
+        expect(captureRequest.headers()['content-type']).toEqual('application/json')
+        expect(captureRequest.headers()['content-encoding']).toEqual('gzip')
+        expect(captureRequest.url()).not.toContain('compression')
         // webkit doesn't allow us to read the body for some reason
         // see e.g. https://github.com/microsoft/playwright/issues/6479
         if (browserName !== 'webkit') {
             const payload = getGzipEncodedPayloady(captureRequest)
-            expect(payload.event).toEqual('$pageview')
-            expect(Object.keys(payload.properties).length).toBeGreaterThan(0)
+            expect(payload.api_key).toEqual('test token')
+            expect(payload.batch[0].event).toEqual('$pageview')
+            expect(Object.keys(payload.batch[0].properties).length).toBeGreaterThan(0)
         }
     })
 
diff --git a/packages/browser/playwright/mocked/retry-queue.spec.ts b/packages/browser/playwright/mocked/retry-queue.spec.ts
index 4ed95affd2..9d7a91181b 100644
--- a/packages/browser/playwright/mocked/retry-queue.spec.ts
+++ b/packages/browser/playwright/mocked/retry-queue.spec.ts
@@ -16,7 +16,7 @@ test.describe('retry queue', () => {
         let successSeen = false
 
         // Mock the capture endpoint to fail initially, then succeed
-        await context.route('**/e/**', async (route) => {
+        await context.route(/\/batch\//, async (route) => {
             const request = route.request()
             captureRequests.push(request)
 
@@ -85,7 +85,7 @@ test.describe('retry queue', () => {
         const captureRequests: Request[] = []
 
         // Mock the capture endpoint to always fail
-        await context.route('**/e/**', async (route) => {
+        await context.route(/\/batch\//, async (route) => {
             captureRequests.push(route.request())
             await route.fulfill({
                 status: 500,
@@ -131,7 +131,7 @@ test.describe('retry queue', () => {
 
         // Mock the capture endpoint to fail initially
         let shouldSucceed = false
-        await context.route('**/e/**', async (route) => {
+        await context.route(/\/batch\//, async (route) => {
             captureRequests.push(route.request())
             if (shouldSucceed) {
                 await route.fulfill({
diff --git a/packages/browser/playwright/mocked/session-recording/session-recording-network-recorder.spec.ts b/packages/browser/playwright/mocked/session-recording/session-recording-network-recorder.spec.ts
index 8971e5abd2..c1832e6b1a 100644
--- a/packages/browser/playwright/mocked/session-recording/session-recording-network-recorder.spec.ts
+++ b/packages/browser/playwright/mocked/session-recording/session-recording-network-recorder.spec.ts
@@ -149,10 +149,7 @@ test.beforeEach(async ({ context }) => {
                 expect(hasEntry(/http:\/\/localhost:\d+\/playground\/cypress\//, 'navigation')).toBe(true)
                 expect(hasEntry(/https:\/\/localhost:\d+\/static\/array.js/, 'script')).toBe(true)
                 expect(
-                    hasEntry(
-                        /https:\/\/localhost:\d+\/array\/test%20token\/config\?ip=0&_=\d+&ver=1\.\d\d\d\.\d+/,
-                        'fetch'
-                    )
+                    hasEntry(/https:\/\/localhost:\d+\/array\/test%20token\/config\?_=\d+&ver=1\.\d\d\d\.\d+/, 'fetch')
                 ).toBe(true)
                 expect(
                     hasEntry(/https:\/\/localhost:\d+\/static\/(lazy-)?recorder.js\?v=1\.\d\d\d\.\d+/, 'script')
diff --git a/packages/browser/playwright/mocked/slim-bundle.spec.ts b/packages/browser/playwright/mocked/slim-bundle.spec.ts
index 0ea0eede64..1d17424694 100644
--- a/packages/browser/playwright/mocked/slim-bundle.spec.ts
+++ b/packages/browser/playwright/mocked/slim-bundle.spec.ts
@@ -89,7 +89,7 @@ test.describe('slim bundle + extension bundles (#3313)', () => {
         })
 
         // Mock the capture endpoint
-        void context.route('**/e/*', (route) => {
+        void context.route(/\/(?:e|batch)\//, (route) => {
             route.fulfill({
                 status: 200,
                 contentType: 'application/json',
diff --git a/packages/browser/playwright/mocked/utils/posthog-playwright-test-base.ts b/packages/browser/playwright/mocked/utils/posthog-playwright-test-base.ts
index fd8a9cb524..0e97435891 100644
--- a/packages/browser/playwright/mocked/utils/posthog-playwright-test-base.ts
+++ b/packages/browser/playwright/mocked/utils/posthog-playwright-test-base.ts
@@ -102,7 +102,7 @@ export const test = base.extend<{
     },
     mockStaticAssets: [
         async ({ context, staticOverrides }, use) => {
-            void context.route('**/e/*', (route) => {
+            void context.route(/\/(?:e|batch)\//, (route) => {
                 route.fulfill({
                     status: 200,
                     contentType: 'application/json',

From 51d74dc7a16ac05151d4a3e49510e4ce9dbc0fe5 Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 17:09:04 +0200
Subject: [PATCH 16/17] test: log batch ingestion in testcafe

---
 packages/browser/testcafe/helpers.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/browser/testcafe/helpers.js b/packages/browser/testcafe/helpers.js
index 0ed8e3df4d..d05d42e099 100644
--- a/packages/browser/testcafe/helpers.js
+++ b/packages/browser/testcafe/helpers.js
@@ -19,7 +19,7 @@ export const {
 
 const HEADERS = { Authorization: `Bearer ${POSTHOG_PERSONAL_API_KEY}` }
 
-export const captureLogger = RequestLogger(/ip=0/, {
+export const captureLogger = RequestLogger(/\/(?:e|batch)\//, {
     logRequestHeaders: true,
     logRequestBody: true,
     logResponseHeaders: true,

From b6951a8b3e96477a3059b4bb2c930bb5dac5f4ca Mon Sep 17 00:00:00 2001
From: Manoel Aranda Neto <marandaneto@gmail.com>
Date: Tue, 19 May 2026 21:10:55 +0200
Subject: [PATCH 17/17] fix: remove unused retry count query param

---
 compliance/browser/adapter.js                 | 11 +----
 .../playwright/mocked/retry-queue.spec.ts     | 46 +++----------------
 .../src/__tests__/posthog-core-also.test.ts   | 26 -----------
 .../browser/src/__tests__/retry-queue.test.ts | 20 +-------
 packages/browser/src/retry-queue.ts           |  7 +--
 5 files changed, 10 insertions(+), 100 deletions(-)

diff --git a/compliance/browser/adapter.js b/compliance/browser/adapter.js
index 8408d2df2d..46cd109f69 100644
--- a/compliance/browser/adapter.js
+++ b/compliance/browser/adapter.js
@@ -79,13 +79,10 @@ global.XMLHttpRequest = function() {
                         }
                     }
 
-                    const urlObj = new URL(requestUrl, 'http://dummy')
-                    const retryCount = parseInt(urlObj.searchParams.get('retry_count') || '0', 10)
-
                     state.requestsMade.push({
                         timestamp_ms: Date.now(),
                         status_code: xhr.status,
-                        retry_attempt: retryCount,
+                        retry_attempt: 0,
                         event_count: events.length,
                         uuid_list: events.map(e => e.uuid).filter(Boolean),
                     })
@@ -153,14 +150,10 @@ global.fetch = async (url, options) => {
                 // Note: Blob bodies (gzipped data) are not parsed
             }
 
-            // Extract retry count from URL if present
-            const urlObj = new URL(url)
-            const retryCount = parseInt(urlObj.searchParams.get('retry_count') || '0', 10)
-
             state.requestsMade.push({
                 timestamp_ms: Date.now(),
                 status_code: response.status,
-                retry_attempt: retryCount,
+                retry_attempt: 0,
                 event_count: events.length,
                 uuid_list: events.map(e => e.uuid).filter(Boolean),
             })
diff --git a/packages/browser/playwright/mocked/retry-queue.spec.ts b/packages/browser/playwright/mocked/retry-queue.spec.ts
index 9d7a91181b..8adf37e047 100644
--- a/packages/browser/playwright/mocked/retry-queue.spec.ts
+++ b/packages/browser/playwright/mocked/retry-queue.spec.ts
@@ -50,28 +50,8 @@ test.describe('retry queue', () => {
             expect(successSeen).toBe(true)
         }).toPass({ timeout: 50000 })
 
-        // Check that we got multiple requests
-        expect(captureRequests.length).toBeGreaterThanOrEqual(3)
-
-        // Verify the first request had no retry_count
-        const firstRequest = captureRequests[0]
-        expect(firstRequest.url()).not.toContain('retry_count')
-
-        // Verify retry_count increments
-        const retryCountMatches = captureRequests
-            .map((req) => {
-                const match = req.url().match(/retry_count=(\d+)/)
-                return match ? parseInt(match[1]) : null
-            })
-            .filter((count) => count !== null)
-
-        // Should see incrementing retry counts
-        expect(retryCountMatches.length).toBeGreaterThanOrEqual(2)
-        expect(retryCountMatches).toContain(1)
-        expect(retryCountMatches).toContain(2)
-        // Verify counts are actually incrementing (not stuck at 1)
-        const uniqueCounts = Array.from(new Set(retryCountMatches))
-        expect(uniqueCounts.length).toBeGreaterThan(1)
+        // Check that we retried the failed requests before succeeding
+        expect(captureRequests.length).toBeGreaterThanOrEqual(maxErrorResponses + 1)
 
         // After success, record the count and verify no more requests arrive
         const requestCountAfterSuccess = captureRequests.length
@@ -80,7 +60,7 @@ test.describe('retry queue', () => {
         }).toPass({ timeout: 5000 })
     })
 
-    test('stops retrying after 10 attempts', async ({ page, context }) => {
+    test('retries failed capture requests without unbounded attempts', async ({ page, context }) => {
         test.setTimeout(60000)
         const captureRequests: Request[] = []
 
@@ -107,23 +87,9 @@ test.describe('retry queue', () => {
         // We'll wait long enough to see at least 3-4 retries
         await page.waitForTimeout(25000)
 
-        // Extract all retry counts
-        const retryCountMatches = captureRequests
-            .map((req) => {
-                const match = req.url().match(/retry_count=(\d+)/)
-                return match ? parseInt(match[1]) : null
-            })
-            .filter((count) => count !== null)
-            .sort((a, b) => a! - b!)
-
-        // Should have some retries but not exceed 10
-        expect(retryCountMatches.length).toBeGreaterThan(0)
-        const maxRetryCount = Math.max(...(retryCountMatches as number[]))
-        expect(maxRetryCount).toBeLessThanOrEqual(10)
-
-        // Verify counts are incrementing
-        expect(retryCountMatches).toContain(1)
-        expect(retryCountMatches).toContain(2)
+        // Should have some retries but not exceed the initial attempt + 10 retries
+        expect(captureRequests.length).toBeGreaterThan(1)
+        expect(captureRequests.length).toBeLessThanOrEqual(11)
     })
 
     test('immediately retries when coming back online', async ({ page, context }) => {
diff --git a/packages/browser/src/__tests__/posthog-core-also.test.ts b/packages/browser/src/__tests__/posthog-core-also.test.ts
index fd2535397e..69f69628ba 100644
--- a/packages/browser/src/__tests__/posthog-core-also.test.ts
+++ b/packages/browser/src/__tests__/posthog-core-also.test.ts
@@ -398,32 +398,6 @@ describe('posthog core', () => {
             requestSpy.mockRestore()
         })
 
-        it('sends retried analytics requests with query params to /batch/', () => {
-            const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
-            const posthog = posthogWith({ ...defaultConfig, request_batching: false })
-            posthog._onRemoteConfig({ supportedCompression: [Compression.GZipJS] } as RemoteConfig)
-            requestSpy.mockClear()
-
-            posthog._send_request({
-                url: 'https://us.i.posthog.com/batch/?retry_count=1',
-                data: { event: 'event-name' },
-                compression: Compression.GZipJS,
-            })
-
-            expect(requestSpy).toHaveBeenCalledWith(
-                expect.objectContaining({
-                    url: 'https://us.i.posthog.com/batch/',
-                    compression: Compression.GZipJS,
-                    _useContentEncoding: true,
-                    data: expect.objectContaining({
-                        api_key: 'testtoken',
-                        batch: [expect.objectContaining({ event: 'event-name' })],
-                    }),
-                })
-            )
-            requestSpy.mockRestore()
-        })
-
         it('sends sendBeacon analytics requests to /batch/', () => {
             const requestSpy = jest.spyOn(requestModule, 'request').mockImplementation(jest.fn())
             const posthog = posthogWith({ ...defaultConfig, request_batching: false })
diff --git a/packages/browser/src/__tests__/retry-queue.test.ts b/packages/browser/src/__tests__/retry-queue.test.ts
index 22efb1dabe..fef06cd5ca 100644
--- a/packages/browser/src/__tests__/retry-queue.test.ts
+++ b/packages/browser/src/__tests__/retry-queue.test.ts
@@ -100,25 +100,7 @@ describe('RetryQueue', () => {
         // clears queue
         expect(retryQueue.length).toEqual(0)
         expect(mockPosthog._send_request).toHaveBeenCalledTimes(4)
-        // Check the retry count is added
-        expect(mockPosthog._send_request.mock.calls.map(([arg1]) => arg1.url)).toEqual([
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-        ])
-    })
-
-    it('adds the retry_count to the url', () => {
-        enqueueRequests()
-        fastForwardTimeAndRunTimer(3500)
-
-        expect(mockPosthog._send_request.mock.calls.map(([arg1]) => arg1.url)).toEqual([
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-            '/e?retry_count=1',
-        ])
+        expect(mockPosthog._send_request.mock.calls.map(([arg1]) => arg1.url)).toEqual(['/e', '/e', '/e', '/e'])
     })
 
     it('tries to send requests via beacon on unload', () => {
diff --git a/packages/browser/src/retry-queue.ts b/packages/browser/src/retry-queue.ts
index 08ef5ac2ff..c873ebff25 100644
--- a/packages/browser/src/retry-queue.ts
+++ b/packages/browser/src/retry-queue.ts
@@ -1,10 +1,9 @@
 import { RetriableRequestWithOptions } from './types'
 
-import { isPositiveNumber, isUndefined } from '@posthog/core'
+import { isUndefined } from '@posthog/core'
 import { logger } from './utils/logger'
 import { window } from './utils/globals'
 import { PostHog } from './posthog-core'
-import { extendURLParams } from './request'
 import { addEventListener } from './utils'
 
 const thirtyMinutes = 30 * 60 * 1000
@@ -68,10 +67,6 @@ export class RetryQueue {
     }
 
     retriableRequest({ retriesPerformedSoFar, ...options }: RetriableRequestWithOptions): void {
-        if (isPositiveNumber(retriesPerformedSoFar)) {
-            options.url = extendURLParams(options.url, { retry_count: retriesPerformedSoFar })
-        }
-
         this._instance._send_request({
             ...options,
             callback: (response) => {